I would check out this forum...
http://www.opticstalk.com/
They are some very helpful people out there... I would say from what I hear the 4200 is better optically but has really poor eye relief for it's price. The Leupold is a great rugged scope with exceptional eye relief, but is really overpriced due to their longstanding reputation of excellent scopes.
By the way the Bushnell 4200 is "really" a Bausch and Lomb... Same specs, now with the rainguard coating.
Also look at Sightron SII's, they compare very favorable to a VX-III as they are the same glass (Sightron makes Leupolds glass). I have a Nikon Monarch UCC and think it is better than my Leupold VX-III for a little less, I think the Leupold looks cleaner though.